Hey everybody!
I had a similar problem with installing mono on Vero with OSMC RC2 and I could resolve it by following the guide here:
HTPC Guide - Install Sonarr
There is a hint there to install libmono-cil-dev by lowering the pin preference, however it refers to the old Debian version “wheezy” → OSMC seems to be based on a newer version of Debian called “jessie”, therefore you have to replace wheezy with jessie.
So, edit the file
sudo nano /etc/apt/preferences
and insert the following
Package: *
Pin: release n=jessie
Pin-Priority: 998
Then update sources and install mono
sudo apt-get update
sudo apt-get install libmono-cil-dev -y
That worked for me. You should remove the above pin preference afterwards however.